Hepatitis B virus (HBV) causes acute hepatitis in millions of people each year. Of those infected, 10% reman chronically infected and have an increased risk of developing cirrhosis and primary hepatocellular carcinoma. The chronic infection cycle might be broken by treatments designed to protect uninfected hepatocytes by passive immunization with antibody to the virus- encoded attachment protein or its receptor on hepatocytes. Unfortunately, the identity of these two target molecules is unknown. The three surface (S) proteins of HBV which are the candidates for the virus attachment protein are found on the infectious virus particles, but also on the noninfectious hepatitis B surface antigen (HBsAg) particles. Evidence is presented that only the HBsAg particles containing the large S protein (large S particles) are able to attach directly to liver plasma membranes or to the hepatoma- derived cell line, HepG2. The HBV large S protein, therefore, is the best candidate for the HBV attachment protein. One goal of this project is to determine the site on the large S protein which contains the attachment function through the use of synthetic peptides representing portions of the large S protein, anti-peptide antibodies, and specific monoclonal antibodies. Once the attachment site on the large S protein is identified, it would be a prime candidate for inclusion in future, more effective, HBV vaccines. However, the major goal of the proposed work is to identify and characterize the molecule which serves as the hepatocyte receptor for HBV. The biochemical nature of the receptor will be determined by treatment of HepG2 cells with proteases, glycosidases, and lipases, and by competition with carbohydrates and lipids. The molecular weight of the receptor will be determined by using the large S particles as a probe for blotting separated plasma membranes proteins and for isolating a receptor-ligand complex. Antibodies to the receptor molecule will be generated by an anti-idiotype scheme or as monoclonal antibodies which bind to the surface of HepG2 cells and prevent large S particles from binding. Such anti-receptor antibodies would be valuable in identifying and characterizing the HBV receptor, and possibly lead to a therapy for chronic HBV infection, as mentioned above. All of the blocking peptides and antibodies described above will be used to confirm that the infectious HBV particles use the same receptor thereby validating its potential as a target for therapy.